The present invention relates to an apparatus and method for controlling combustion in internal combustion engines, and more particularly, to an apparatus and method for controlling combustion in an internal combustion engine that injects fuel directly into its cylinders and performs both stratifed charge combustion and homogeneous charge combustion.
In a typical automotive engine, fuel is injected into an intake port by a fuel injection valve to charge the associated combustion chamber with a homogeneous mixture of fuel and air. The air fuel ratio of the mixture is stoichiometric. An air intake passage is opened and closed by a throttle valve, which is operated in cooperation with an acceleration pedal. The opening of the throttle valve adjusts the intake air amount (and ultimately the amount of homogeneously mixed air and fuel) that is supplied to the combustion chambers of the engine. This controls engine power.
To improve fuel efficiency, lean combustion may be performed. When performing lean combustion, the air fuel ratio of the homogeneous mixture drawn into the combustion chamber is increased to a value greater than the stoichiometric air fuel ratio. The lean air fuel mixture is then swirled and burned. Japanese Unexamined Patent Publication No. 8-4568 describes an apparatus for controlling combustion in an engine that performs such lean combustion.
The apparatus first determines a basic fuel injection amount in accordance with the depression degree of the acceleration pedal. The apparatus then corrects the basic fuel injection amount to obtain an actual fuel injection amount, which is the actual amount of injected fuel, in accordance with the difference between the air fuel ratio of the mixture supplied to the engine and the maximum air fuel ratio permitted for stable combustion (lean limit). This enhances fuel efficiency while preventing uncomfortable engine torque fluctuations (power fluctuations).
However, when performing homogeneous charge combustion, the throttling action of the throttle valve drastically decreases the pressure in the intake passage regardless of whether the air fuel ratio of the mixture corresponds to the stoichiometric value or to the lean limit value. This increases energy loss due to pumping (pumping loss) and decreases efficiency. Stratified charge combustion solves this problem. In stratified charge combustion, the throttle valve is opened wide, and fuel is supplied directly into each combustion chamber. This delivers a rich, highly combustible air-fuel mixture to the vicinity of the spark plug and improves ignition.
Stratified charge combustion is performed when the engine load is small. The concentration of fuel in the vicinity of the spark plug further increases fuel efficiency, and pumping loss is reduced. When the engine load increases, the combustion mode is shifted from stratified charge combustion to lean homogeneous charge combustion. During lean homogeneous charge combustion, the fuel injected from the fuel injection valve is dispersed homogeneously in the cylinder.
In such an engine that shifts combustion modes, it is desirable that the engine torque fluctuation be as close as possible to a target value when performing stratified charge combustion just as when performing lean homogeneous charge combustion. However, when performing stratified charge combustion, a rich mixture is delivered to the vicinity of the spark plug. Thus, it is difficult to minimize the difference between the torque fluctuation and its target value just by computing the actual fuel injection amount by correcting the basic fuel injection amount in the same manner as when performing lean homogeneous charge combustion.
In the prior art, this problem is dealt with by installing an exhaust gas recirculation (EGR) mechanism, which reduces engine emissions, in the engine to minimize the difference between the torque fluctuation and the target value when performing stratified charge combustion. The EGR mechanism includes an EGR passage, which connects an engine exhaust duct with an air intake passage, and an EGR valve for opening and closing the EGR passage. If the engine load is small, the EGR valve is opened to recirculate the exhaust gas. This enhances the reduction of NOx.
When the torque fluctuation is greater than the target value, the EGR amount is reduced to decrease the torque fluctuation to a value lower than the target value. When the torque fluctuation is smaller than the target value, the EGR amount is increased to minimize the difference between the torque fluctuation and the target value. Thus, the increase or decrease of the EGR amount during stratified charge combustion minimizes the difference between the engine torque fluctuation and the target value. The correction of the basic fuel injection amount is not carried out when performing stratified charge combustion. The basic fuel injection amount, which is obtained in accordance with the depression degree of the acceleration pedal, is directly used as the actual fuel injection amount.
More specifically, as shown in FIG. 10, if the engine load (which is represented by the basic fuel injection amount Q.sub.f0) is in a stratified charge combustion range, the actual fuel injection amount Q.sub.f is equal to the basic fuel injection amount Qf.sub.0, which is obtained in accordance with the acceleration pedal depression degree. Thus, in the stratified charge combustion range, as the acceleration pedal depression degree changes, the actual fuel injection amount Q.sub.f and the basic fuel injection amount Qf.sub.0 are directly proportional as indicated by solid line L1. When the engine load enters a lean homogeneous charge combustion range, the actual fuel injection amount Q.sub.f is obtained by correcting the basic fuel injection amount Q.sub.f0. Thus, as the acceleration pedal depression degree changes, the actual fuel injection amount Q.sub.f and the basic fuel injection amount Q.sub.f0 shift as indicated by dotted line L2 or L3. As apparent from the graph of FIG. 10, at the point of shifting of the combustion mode, a sudden increase or decrease occurs in the actual fuel injection amount Q.sub.f. This produces a torque shock, which degrades the performance of the engine.